Modern wireless telecommunication systems need to guarantee transmissions with data transfer rates as high as possible; at the same time, they need to be designed in such a way to be cost-effective and to require low power consumptions.
For these reasons, systems based on the homodyne architecture are nowadays extensively exploited. Homodyne systems are based on a so-called direct conversion method, according to which a receiver directly converts the received signals from a carrier Radio Frequency (RF) to a base-band frequency. In this way, only one mixer stage is usually required in the receiver, thereby resulting in lower power consumption and easier implementation of the receiver in an integrated form. Thereby, homodyne systems allow avoiding the use of expensive intermediate-frequency filters, which are instead required in the heterodyne architectures.
Among the various known homodyne systems, an important class thereof includes the ones based on quadrature transmitters. As it is well known to those skilled in the art, a quadrature transmitter usually includes one or more quadrature modulators, which receive two base-band signals to be transmitted and generate a corresponding modulated signal. More particularly, the base-band signals are brought to the RF domain used for the transmission by properly modulating them with a corresponding carrier wave, so that the resulting signals are in quadrature to each other; then, the signals in quadrature are mixed together to form the modulated signal.
For example, the base-band signals of a typical quadrature homodyne architecture may be generated exploiting the so-called Orthogonal Frequency-Division Multiplexing (OFDM) technique, which is a particular digital modulation scheme that makes use of a large number of closely-spaced sinusoidal waves; likewise, the base band signals may by generated by filtering Binary Phase-Shift Keying (BPSK) sequences using Nyquist shapes or by exploiting other known techniques.
However, the occurrence of imperfections in real quadrature modulators, such as gain and phase imbalances, generates corresponding gain and phase errors in the modulated signal; the gain and phase errors may have a detrimental effect on the system performance. Compensation for these errors, either with digital signal processors or analog circuits, is essential in order to meet the stringent out-of-band emission requirements of modern wireless telecommunication systems.
In order to mitigate this problem, a possible solution is of pre-compensating the base-band signals to be modulated so as to counterbalance the gain and phase errors; for this purpose a proper compensation is applied to the base-band signals, which compensation is quantified according to an estimate of the gain and phase errors.
For example, the document “New Methods for Adaption of Quadrature Modulators and Demodulators in Amplifier Linearization Circuits” by J. K. Cavers discloses a method for compensating the gain and phase errors of quadrature modulators, which method make uses of a closed feedback measure loop. In particular, the method provides for feeding the quadrature modulator with a base-band test signal, which is modulated to obtain a corresponding RF signal; then, the RF signal is provided to an envelope detector, in such a way to obtain an indication of the gain and phase errors generated by the imperfections of the modulator. However, the output of the modulator is phase-dependant, since it depends on the test signal; thus the method requires the exact knowledge of a measure of the loop's delay, which is usually not available. An alternative solution provided by the method consists of the transmission of a dc tone in the base-band and then in the performing of measures with a “step and measure” technique. This last operation cannot be easily performed in OFDM systems, since the dc tone—corresponding to the frequency of the carrier—is usually removed by the circuits adapted to couple the base-band section with the RF section.
A different approach is disclosed in the document “A WLAN Direct Upconversion Mixer with Automatic Calibration” by Jan Craninckx, Bjorn Debaillie, Boris Come and Stephane Donnay. According to this approach, the modulated signal generated by the modulator is fed back to the inputs of the modulator, and the output thereof is used for estimating the compensation. The main drawback of this approach regards its hardware implementation, since a measure loop used for this approach has to include switches that need to be perfectly insulated.